1. Field of the Invention
This invention relates generally to a corona discharge igniter for emitting a non-thermal plasma to ignite a mixture of fuel and air of a combustion chamber, and methods of manufacturing the same.
2. Description of the Prior Art
An example of a corona discharge ignition system is disclosed in U.S. Pat. No. 6,883,507 to Freen. In the corona discharge ignition system, an electrode of an igniter is charged to a high radio frequency (“RF”) voltage potential, creating a strong RF electric field in the combustion chamber. The electric field causes a portion of the fuel-air mixture in the combustion chamber to ionize and begin dielectric breakdown, facilitating combustion of the fuel-air mixture. However, the electric field is controlled so that the fuel-air mixture maintains dielectric properties and corona discharge occurs, also referred to as a non-thermal plasma. The electric field is controlled so that the fuel-air mixture does not lose of all dielectric properties, which would create a thermal plasma and an electric arc between the electrode and grounded cylinder walls or piston. The current of the corona discharge is small and the voltage potential at the electrode remains high in comparison to an arc discharge. The ionized portion of the fuel-air mixture forms a flame front which then becomes self-sustaining and combusts the remaining portion of the fuel-air mixture.
The electrode of the corona discharge ignition system is typically formed of an electrically conductive material extending from an electrode terminal end to an electrode firing end, and an insulator including a matrix of electrically insulating material extends along the electrode. The igniter of the corona discharge ignition system does not include any grounded electrode element in close proximity to the electrode. Rather, as alluded to above, the ground is provided by the cylinder walls or piston of the internal combustion engine. An example igniter is disclosed in U.S. Patent Application Publication No. US 2010/0083942 to Lykowski and Hampton.
For internal combustion engine applications, it is typically preferred that the non-thermal plasma formed includes multiple streams of ions in the form of a corona discharge. The streams ignite the air-fuel mixture along the entire length of the streams, throughout the combustion chamber, and thus provide a robust ignition. As discussed in the Freen patent, the electric field is preferably controlled so that the corona discharge does not proceed to an electron avalanche which would result in an arc discharge from the electrode to the pounded cylinder wall or piston. Under certain conditions, such as when voltages above a certain threshold are applied to the igniter, the density of the ions increases and the arc discharge may be formed. The arc discharge comprises a single stream of ions, rather than the desired plurality of streams. The arc discharge occupies a much smaller space in the combustion chamber than the corona discharge and thus can reduce the quality of ignition.